1. Field of the Invention
This invention relates to portland cement compositions with and without added gypsum. More particularly, this invention relates to portland cement compositions and products made therefrom wherein carbon dioxide is added alone or in combination with acids or acid/salts during mixing with water to maintain fluidity of the resulting aqueous cement or concrete slurry, to minimize cement shrinkage during drying and atmospheric carbonation, and to control setting of the cement.
Ground portland cement clinker is universally mixed in the cement plant with about 4% to 8% of finely ground gypsum (calcium sulfate hydrate) to prevent an almost instantaneous thickening of the ground cement particles which would result when water is added. The portland cement mix containing the cement, gypsum, aggregates and about 40 to 85 parts water per 100 parts cement progressively loses fluidity as the cement hydrates, but is suitably handleable for one to several hours. Reduction in water levels results in progressively higher concrete strengths but requires significantly reduced aggregate levels to maintain satisfactory fluidity. The use of added vibration or water reducers such as, lignosulfonates, permits somewhat reduced water levels and thus increased strengths.
The amount of gypsum added is determined by each cement manufacturer to properly control the rate of hydration for its particular chemical composition and fineness. Portland cement specifications, however, limit the amount of gypsum (expressed as SO.sub.3) which a cement manufacturer can add, depending somewhat on the chemical composition of its clinker. The manufacturer is not required to add gypsum, but instead is limited in how much can be added; and at present has no feasible alternate. The reason a limitation is put on the upper amount of gypsum which is permitted is that concrete made with too much gypsum instead of shrinking will expand in time and break up.
The added gypsum in portland cement apparently exerts its retarding action by interacting with the tetracalcium aluminate hydrate which immediately forms when water is added to the cement grains. The molar volume of the anhydrous tricalcium aluminate increases four fold in forming the hydrate and doubles again in forming the aluminate trisulfate hydrate complex. This voluminous complex on the surface of the cement grains is believed to slow down the rate of hydration substantially. The hydration does continue slowly, however, and progressively reacts with the remaining soluble gypsum to form more aluminate trisulfate hydrate until within a few hours almost all the gypsum has been reacted. The cement then starts to set as more aluminum hydrate is formed and is released into the solution where it interacts with the aluminate trisulfate hydrate to form the aluminate monosulfate hydrate with less than half the molar volume of the sulfate. These volume changes which occur in forming aluminate trisulfate hydrate can be a factor in increased viscosity of cement slurries and in dimensional changes and cracking in cement products. Thus, it is desirable to find a replacement for gypsum which will improve fluidity, retard setting, and does not result in hydration products of such large volume change.
2. The Prior Art
This invention is to be distinguished from those processes which treat already molded and set cements and concrete products with carbon dioxide primarily for the purpose of accelerating hardening and preventing carbon dioxide shrinkage. It is also distinguished from those processes where hardened and dried concrete products are reacted with carbon dioxide under pressure to obtain higher strength properties. It is also to be distinguished from the reaction of cement compacts containing little or no water which are compressed together at high pressure and then subjected to carbon dioxide under pressure at elevated humidities that develop some internal strength.
It is widely recognized that contamination with just fractions of a percent of alkali metal carbonate when added to gypsum-containing ground portland cement or to the mixing water will result in set up of the fluid cement slurry within minutes after mixing of cement and water. Likewise, atmospheric exposure of ground cement for extended periods at high humidity is known to modify the cement so that it also will lose fluidity shortly after mixing with water. The latter difficulty is related to low level carbonation which can occur on the surface of the cement particles at high humidity.
Since carbonate contamination causes the cement or concrete slurry to become immobile or nearly so within minutes rather than in hours in usual setting up of portland cement and concrete products, the industry takes particular precautions to avoid such contamination. When such contamination occurs and cement and concrete slurries become immobile within minutes, it may be possible with increased mechanical action or dilution to refluidize the concrete and still be able to use it. Such a phenomenon is thus called a "false set" as compared to the normal extended set after which time possible refluidization for use is not feasible.
As it is widely known that false set can result from contamination with alkali metal carbonates or prior atmospheric contamination of cement products and serious operational and handling problems thereby resulted, the portland cement industry has rigorously avoided carbon dioxide and alkali metal carbonates. Only recently has it been demonstrated that if non-gypsum containing portland cement powders were used that the proper combination of sulfonated lignins or lignosulfonates with sodium carbonate (U.S. Pat. No. 3,689,294) or better with sodium bicarbonate (U.S. Pat. No. 3,960,582) permitted the formulation of fluid, very low water cements which resulted in very high strength. No known work has been reported on the beneficial utilization of carbon dioxide during the mixing of portland cement products with or without gypsum.
Whereas it has been known for many years that prior carbon dioxide or carbonate contamination can cause severe problems in mixing and handling fluidity, only in the past ten to twenty years has it become progressively realized that atmospheric carbonation during drying of thin or porous cement and concrete products can be the cause of up to half of the very detrimental shrinkage and cracking previously attributed just to atmospheric drying. The carbon dioxide in the humid atmosphere progressively penetrates cement products during drying and apparently reacts with the calcium hydroxide associated with the calcium aluminate, ferrite, and silicate hydrates and with the free calcium hydroxide crystals. Eventually about 45 parts of carbon dioxide can actually combine with 100 parts of cement. As the carbon dioxide penetrates the cement product, the pH of the cement is progressively lowered from the very alkaline pH 12 - 13 to below pH 10; and the depth penetrated can be determined using phenophthalin indicator solution on a cement section.
In the practice of this invention, the fluid portland cement slurries with and without gypsum present are homogeneously carbonated with generally only about one one-hundredth to one tenth as much carbon dioxide as can actually react with the hydrated cement. With these lower levels of homogeneous carbonation in this invention, the pH of the cement remains about the same as for normal cements.
It is, therefore, the general object of this invention to provide new portland cement compositions.
Another object of this invention is to provide portland cement mixes having a controlled setting time and stabilized upon hydration using carbon dioxide.
A further object of this invention is to provide fluid portland cement mixes with or without added gypsum which are stabilized upon hydration with carbon dioxide and include an accelerator for hardening.
Still another object of this invention is to provide a set regulated portland cement mixture having reduced water requirements and yet remain free-flowing until setting.
Other objects, features and advantages of this invention will become evident from the following detailed description of the invention.